1. Field of the Invention
This invention relates to an apparatus and method for inspecting the surface quality of objects having smooth surfaces, ranging from highly reflective to lowly reflective for defects. More particularly, this invention relates to an apparatus and method whereby parts with smooth surfaces, whether highly reflective or lowly reflective can be inspected by observing a beam of light that is reflected from the part.
2. Description of the Prior Art
The inspection of parts having smooth surfaces has been a long outstanding problem and is of particular importance to the automotive industry. It is known to use a special lighting arrangement in conjunction with pretreating the part with highlighting oil which improves the reflective property of the surface. Unfortunately, the use of the highlighting oil is disadvantageous because the oil has a mild toxicity and unnecessary time is expended in applying the oil and subsequently removing it after the inspection has been completed. This method of inspection is carried out at a station often referred to as the `Green Room`. This room has special lighting and a special dolly is provided to hold the sample. The part to be inspected is cleaned and the highlighting oil is spread over the surface. The oil is usually allowed to settle for ten minutes and then an inspector moves about the sample looking for defects. This process does not lend itself to further adaptation for inspection through machine vision systems. Also, it is unsuitable for assembly line inspection. Since the inspection process takes a great deal of time, only a small portion of the total number of parts being produced can be inspected. If a defect is noted, the production line is stopped but, by that time, many parts containing the same defect usually have already been produced. It is not unusual for this type of inspection to take more than two hours for one part.
Another type of inspection system is described in U.S. Pat. No. 3,666,370 entitled "Strip Flatness Inspection Method", naming E. F. Seasholtz as inventor and issued on May 30, 1972. This patent describes the use of a diffused light source to inspect steel sheet or strip in a continuous processing line. The diffused light source can be a fluorescent bulb or bulbs mounted in a shallow rectangular box. The box is said to have a plurality of removable shutters for adjusting the height and/or thickness of a light slit which by means of a white translucent cover is transmitted in the form of a diffused light impinging on the strip being inspected and reflected to the eye of an observer (whether human or a camera). The slit and a plane of the surface being inspected are parallel to one another. This system has not replaced the `Green Room` system of inspection, described above, in the automotive industry. Further, this system does not produce acceptable results for parts having lowly reflective surfaces or highly reflective surfaces when compared to the results achievable with the present invention. It should be noted that the translucent cover is shown in FIGS. 1 and 2 as being located at the front of the box and would therefore have the effect of scattering the light from the box in all directions, thereby destroying the effect of any gradient of intensity previously created in the light. The effect of placing the cover at the front of the box is essentially the same as if a flourescent lightbulb was placed at the front of the box.
With known inspection systems, especially for highly reflective surfaces, the angle of reflection within which an observer can detect defects is much too narrow and the area of inspection is much too small.